This invention relates to an apparatus and a method for checking the attitude of a vehicle.
Checking the attitude of a vehicle is important to optimize road grip, driving comfort and tyre wear.
In effect, road grip and tyre wear depend on the adherence of the vehicle to the road surface which is in turn principally the result of the following two factors: the area of wheel contact with the road surface and wheel drift; both of these factors depend on the geometry of the vehicle's chassis and suspension.
The geometry of a vehicle chassis is defined precisely by the characteristic parameters of the vehicle's attitude, including the characteristic wheel angles, wheel track and pitch and other parameters such as suspension length. The correct values of these parameters are specified by the vehicle manufacturer and are usually variable according to the vehicle type and model.
In the light of this, checking the attitude of a vehicle involves measuring the real values of the above mentioned parameters in such a way that they can, if necessary, be modified and set to the correct values.
Thus, the devices used to check vehicle attitude are based on a suitable system for measuring the quantities which the characteristic attitude parameters depend on.
The quantities measured are then transmitted to a processor which uses appropriate mathematical and geometrical algorithms to calculate the characteristic wheel angles and, if necessary, other attitude parameters, compares them with the correct values for the vehicle being checked (the correct values, supplied by the vehicle manufacturer, are stored in a data base) and calculates and makes available to the operator the values measured and any corrections to be applied to the vehicle to make the characteristic parameters return within correct, specified ranges.
Thus, vehicle attitude checking apparatuses comprise measuring systems, or measuring means, for measuring the values of the parameters representing the geometric and positional characteristics of a wheel of the vehicle relative to a spatial reference.
At present, the measuring systems used can be grouped into the following two categories: those that take measurements (of the parameters representing the geometric and positional characteristics of a wheel) by direct contact with the wheel and those that take the measurements without direct contact with the wheel.
The measuring systems belonging to the first category generally comprise a plurality of measuring heads, each of which is designed to interact mechanically with a vehicle wheel and is equipped with suitable angular transducers, of mechanical or electronic type, capable of detecting the position and orientation of the head relative to a predetermined spatial reference system. The data measured may be transmitted to the processor by a cable or through a wireless system, for example by a radio or infrared system.
In the measuring systems belonging to the second category, the measuring heads are substituted by measuring instruments, generally of optoelectronic type, based on the capture and subsequent processing of images of the wheel or of a target associated with it, using one or more cameras.
These optoelectronic measuring instruments are designed to detect the position of a suitable target associated with the wheel, in such a way as to determine the equation of the tangent plane and of the wheel axis relative to a reference system integral with the measuring instrument.
Once the relation between the reference systems of the measuring instruments associated with the wheels is known, these systems are able to calculate the position and mutual orientation of the various wheels and to obtain from these the characteristic wheel angles and the other attitude parameters.
The targets are generally suitably shaped material bodies (for example, panels having an outside surface with predetermined properties) which are fixed to the vehicle wheels before the measurements are taken, or they may be generated by projecting laser beams or structured light beams on the vehicle wheels in such a way as to create plain lines going through the wheel or more complex, suitably encoded shapes (light tracks).
Other known optoelectronic measuring instruments do not use any type of encoded target and, instead, identify in the images captured by the cameras the position of actual lines on the wheels, such as, for example, the edge separating the rim from the tyre. A measuring instrument of this type is described in European patent application number EP0895056 in the name of the same Applicant as this invention.
Other optoelectronic measuring systems that do not require the use of targets associated with the wheel are those which use the three-dimensional imaging technology based on associating a depth measurement (Z-axis) with each pixel of the two-dimensional image captured by the camera (X- and Y-axes). These measuring systems can recognize the spatial position of the entire wheel relative to a reference system associated with the measuring system and, knowing the relation between the reference systems associated with the various measuring systems, can derive the relative orientations and positions between the wheels.
The three-dimensional imaging technology makes it possible not only to extract from the two-dimensional image in the X-Y reference system the characteristic parameters of interest (also referred to as “features”), but also to measure accurately the distances along the Z-axis between the image sensor and the object to be measured.
The sensors which use three-dimensional imaging technology are able to identify objects in three-dimensional space at a rate of more than 30 images per second (or 30 frames per second, abbreviated as FPS), allowing an adequate rate of updating the measured vehicle attitude data. These sensors require reception of light rays of known wavelength and which are reflected by the object to be measured in three-dimensional space. Other three-dimensional image sensors measure the distance in different ways, for example using the time of flight (TOF) of the light radiation or by processing information relating to the luminosity of the image received by the sensor.
Whatever the measuring method used, the optoelectronic measuring instruments used are normally mounted on suitable fixed structures from which they can locate the targets associated with the vehicle wheels or on portable structures that can be moved by the operator prior to measurement.
Portable systems usually comprise four units positioned near the wheels to be measured in such a way that they can see each other and determine their relative positions. This is essential, in systems of this kind, to place all the measuring instruments in suitable positions relative to the wheels of the vehicle system to be measured.
Also known in the prior art (as disclosed for example in patent document U.S. Pat. No. 6,456,372) are intermediate structures where the measuring instruments are movably mounted on fixed structures in such a way that their relative positions can be varied according to the size of the vehicle to be measured. For example, in patent document EP0895056 mentioned above, the measuring instruments are slidably mounted on a vehicle lift.
Measuring units mounted on self-propelled units are also known. These units move independently on the floor, following variable paths, in order to perform the operations necessary to determine the vehicle's attitude. Structures of this type are described in patent application number WO2009056392 in the name of the same Applicant as this invention.
The systems currently adopted briefly mentioned above, have some drawbacks, however.
In particular, the fixed or semi-fixed structures have the disadvantage of being rather cumbersome and thus occupying a lot of workshop space dedicated only to vehicle attitude adjustments.
These difficulties are made even worse when the vehicle to be checked is very large, as in the case, for example, of a lorry.
The disadvantage of mobile structures, on the other hand, is that before performing the measurement they have to be suitably positioned by the operator around the vehicle, with possible positioning errors appreciably lengthening working times. Moreover, the measuring units used must be equal in number to the number of wheels on the vehicle, to be checked simultaneously; typically there must be four units. These structures, too, have to be put away after use.
Self-propelled units which transport the measuring instruments independently require an adequate structure for moving the measuring instrument and processing the correct paths to be followed under all working conditions. Further, these self-propelled units must be provided with powerful batteries enabling them to work independently for sessions long enough to measure the attitude of several vehicles without stopping and such batteries considerably increase the weight and overall cost of a self-propelled unit.
Another drawback common to all the solutions mentioned above is the need to limit the working area in which to perform attitude measurements to a predetermined space. Thus, the measuring units (whether fixed or mobile) must always have a common spatial reference system which, typically, is permanently installed inside the working area.
In practice, all the attitude checking systems currently used require fixed structures to be installed in the working area for correcting attitude parameters. The fixed structures consist of vehicle lifts or, alternatively, pits, depending on the size and weight of the vehicles.
Statistical analysis of vehicle attitude parameter measurements, however, show that adjustments are necessary only on a limited number of vehicles, whereas the attitude parameters of most vehicles fall within acceptable limits.
Thus, to reduce total attitude measurement time, it would be desirable to have a measuring system or method capable of preventively selecting the vehicles that do not require adjustment.
In particular, the system described in patent document WO2009056392 has the drawback of being complicated and costly; in fact, it requires means for allowing the mobile unit to be moved automatically along a predetermined path.
This also brings other disadvantages mentioned above, regarding the fact that the system can only be used in a limited, predetermined area.